1. Field of the Invention
This invention relates generally to the wavelength-locking of optical sources such as lasers, and more specifically to locking the sources at wavelengths which are separated from each other by a fixed amount.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3, OC-3, or equivalent connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of optical fibers and protocols such as SONET have been developed for the transmission of data over optical fibers.
Fiber optic communications systems utilize a number of basic building blocks. For example, almost all, if not all, fiber optic systems include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and transmits the resulting optical data signal across the optical fiber to the receiver. The receiver recovers the original data from the received optical data signal. Other basic building blocks include optical amplification, add-drop multiplexing, wavelength filtering, and wavelength stabilization and wavelength locking of sources.
As one example of wavelength locking, heterodyne receivers typically will require the wavelength locking of two optical signals. These receivers typically have better noise performance than direct detection receivers but they require the use of an optical local oscillator. The optical local oscillator is mixed with the incoming optical data signal. This effectively frequency shifts the optical data signal from its original optical carrier frequency down to the difference frequency between the optical data signal and the local oscillator. Typically, the result is an electrical RF signal, which is then processed to recover the data. For efficient operation, the difference frequency between the two optical signals should be held fairly constant. In other words, the optical local oscillator should be wavelength-locked to the optical data signal with an approximately constant frequency offset.
In some of the examples given below, each of the two optical signals has a wavelength of approximately 1.55 micron with a frequency offset of approximately 11.55 GHz between the two signals. In these examples, the 1.55 micron wavelength is selected because the example application is a fiber optic communications system which uses this wavelength. The 11.55 GHz offset is selected because the example application is designed to use this frequency offset in order to recover the data transmitted over the fiber. For further details on such a system, see co-pending U.S. patent application Ser. No. 09/474,659, xe2x80x9cOptical Communications System Using Heterodyne Detectionxe2x80x9d, by Ting K. Yee and Peter H. Chang, filed Dec. 29, 1999; and U.S. patent application Ser. No. 09/728,373, xe2x80x9cxe2x80x9cOptical Communications System Using Multiple Heterodyne Detectionxe2x80x9d, by Ting K. Yee and Peter H. Chang, filed Nov. 28, 2000. However, these examples are not meant to limit the invention. For example, heterodyne receivers may also be used for free space optical communications (e.g., in satellite communications), optical data storage and recognition, or coherent imaging and holography. The wavelengths and frequency offsets required from the wavelength locker in each of these applications will depend on the specifics of the application.
One approach to wavelength locking uses conventional phase locked loop principles to phase lock the actual frequency difference between the two optical signals to a stable reference sinusoid. This approach typically requires phase/frequency detectors and a source for generating the stable reference sinusoid but these components can be both complex and expensive. In addition, phase locked loops are used primarily to lock signals which are fairly pure sinusoids and to lock the signals with a high degree of accuracy (e.g., to actually lock them in phase). Many wavelength-locking applications, including the heterodyne receiver application discussed above, simply do not fit this profile. For example, in the heterodyne application, the incoming optical data signal is an optical carrier which has been modulated with data so it will not be a pure sinusoid and, as a result, the difference frequency signal to be locked to the reference sinusoid likely also will not be a pure sinusoid. In addition, the local oscillator in heterodyne receivers typically does not have to be locked to the optical data signal with the accuracy intended by phase locked loops. Thus, the super-accurate locking simply adds unnecessary expense and complexity. This is compounded by the fact that many fiber optic systems will require large numbers of the wavelength locking device.
Another approach to wavelength locking which has been attempted is based on gas cells. Many of these approaches try to take advantage of the physical properties of various gases. Thus, through non-linear optical or other physical interactions in the gas, two optical signals are locked to each other or two optical signals at slightly different wavelengths are generated from a common source. However, these approaches typically rely on the physical properties of gases and, therefore, are limited in their applicability. For example, if a particular gas has physical properties which allow the locking of xcex1 to xcex2, this is fine if the two wavelengths of interest are xcex1 and xcex2 but is irrelevant if any other wavelengths are desired. It can be especially problematic if it is desirable to tune the wavelength locking over a range of wavelengths and a range of frequency separations, as is often the case. In addition, these approaches require the use of a gas cell. The handling of these cells and the gases used to fill them adds expense and complexity.
Thus, there is a need for approaches to wavelength locking of optical signals which are simple, inexpensive and matched in performance to the requirements of the application, particularly since many applications will require a large number of wavelength-locking devices.
In accordance with the present invention, a device is used for wavelength locking a wavelength-variable optical signal to a target wavelength. The target wavelength is offset from a wavelength of an optical reference signal by a preselected frequency offset. The device includes the following components coupled in series: a photomixer section, a frequency filter and comparison circuitry. The photomixer section includes a square law detector, such as a photodiode. The photomixer section mixes the wavelength-variable optical signal with the optical reference signal to produce a beat component and also produces a frequency test signal from the beat component. When the wavelength-variable optical signal is at the target wavelength, the frequency test signal will be located at a target frequency. The frequency filter has a transfer function which varies monotonically (i.e., either increasing or decreasing) around the target frequency. The frequency filter applies a frequency-dependent gain to the frequency test signal to produce an amplitude test signal. When the frequency test signal is located at the target frequency, the frequency filter will apply a target gain. The comparison circuitry compares the amplitude test signal with an electrical reference signal to determine whether the frequency filter applied the target gain. If it did, then the wavelength-variable optical signal must be at the target wavelength. Based on the determination, the comparison circuitry generates a error signal for adjusting the wavelength of the wavelength-variable optical signal.
This approach has significant advantages compared to traditional approaches, such as those based on phase locked loops. As one example, the circuitry required to implement this approach is significantly simpler than a corresponding phase locked loop. Traditional phase locked loops require a stable sinusoidal reference as well as phase/frequency detectors. In contrast, this approach uses frequency filters and amplitude detectors, which are much simpler devices. Another advantage of this approach over phase locked loops is that this approach is more suitable for handling optical signals which are spectrally broad. For example, in the heterodyne detection application, the optical reference signal is an optical carrier which has been modulated with data and will not be a pure sinusoid. Phase locked loops are primarily designed for very accurate locking of pure sinusoids. Hence they typically will have difficulty with xe2x80x9cdirtyxe2x80x9d signals such as the one just described. In contrast, this approach is well suited for such signals because the frequency filter essentially averages out the effects of a broad spectral signal.
In one embodiment, the frequency test signal includes the beat component and the target frequency equals the preselected frequency offset. In an alternate embodiment, the photomixer section also includes an electrical mixer for frequency shifting the beat component by a fixed amount, for example to an intermediate frequency. The target frequency is correspondingly frequency shifted by the same amount.
In another embodiment, the wavelength-locking device also includes a power divider and an electrical reference arm. The power divider is coupled between the photomixer section and the frequency filter. It splits the frequency test signal into two frequency test signals. One is filtered by the frequency filter. The other is used by the electrical reference arm to generate the electrical reference signal.
In a further refinement of this embodiment, the comparison circuitry includes a pair of matched log amplitude detectors coupled to a differential amplifier. One log amplitude detector is also coupled to the frequency filter and is used to detect a log of an amplitude of the amplitude test signal. The other log amplitude detector is coupled to the electrical reference arm and is used to detect a log of an amplitude of the electrical reference signal. The differential amplifier has two inputs and compares the amplitude of the signals received at the two inputs. The comparison circuitry also includes an adjustable voltage source, which provides an adjustable offset for the differential amplifier.
The use of log amplitude detectors in combination with the adjustable voltage source is advantageous. The voltage source allows an offset to the differential amplifier to be adjusted, which in turn adjusts the target gain, the target frequency and ultimately the frequency offset of the target wavelength. The use of log amplitude detectors allows this offset to be maintained regardless of the amplitude of the incoming optical signals or their beat component.
In further accordance with the invention, a method for wavelength locking a wavelength-variable optical signal to a target wavelength, where the target wavelength is offset from a wavelength of an optical reference signal by a preselected frequency offset, includes the following steps. The optical reference signal and the wavelength-variable optical signal are both received and mixed together to produce a beat component. A frequency test signal is produced from the beat component. When the wavelength-variable optical signal is at the target wavelength, the frequency test signal will be located at a target frequency. A frequency-dependent gain is applied to the frequency test signal to produce an amplitude test signal. In the vicinity of the target frequency, the gain applied varies monotonically with frequency. When the frequency test signal is located at the target frequency, the frequency filter will apply a target gain. An electrical reference signal is received and compared with the amplitude test signal to determine whether the frequency filter applied the target gain. An error signal is generated based on this determination.